To ensure high availability and operational safety of the electrical power supply and to guarantee personnel safety in the operating area of electrical installations, power supply networks are widely used whose active components are separated from the ground potential in contrast to grounded network types. In this kind of power supply network, called ungrounded IT system, an active conductor may exhibit an insulation fault without making it necessary to interrupt the ongoing operation of the installation because no closed circuit can form in this first fault case owing to the ideally infinitely high impedance value between the conductor and ground. In this context, an insulation fault is a faulty state of the IT system that causes the insulation resistance to drop below the admissible insulation level.
Apart from these ungrounded IT systems, another type of network exists worldwide in which a neutral point is connected to ground via sufficiently high impedance. Said impedance can be designed as an ohmic resistance and can be configured to have high or low resistance. In case of the high-resistance grounding considered here, known internationally as HRG, said neutral-point resistance limits an occurring fault current to a value that does not yet cause an overcurrent protection device to be triggered.
Oftentimes, HRG systems of this kind are configured in such a way that the fault current is limited to values between 5 A and 10 A in case of a first insulation fault and the HRG system does not shut down unless the fault current rises above 10 A. Thus, a limitation of the fault current to values of 300 mA, which is common in many European grounded power supply systems for fire protection purposes, is not possible in an HRG system.
Starting from a fault current limited to 5 A, an active power of about 1.8 kW is generated at the current-limiting resistance of the HRG system in a 600 V three-phase network with a dead ground fault of one phase against ground. It appears as if the realization that a risk of fire becomes likely starting at active power values above 60 W, i.e. even with much lower fault currents due to high-resistance insulation fault resistances, has not been adequately considered by the advocates and operators of HRG systems.
The use of insulation monitoring devices, which are common in IT systems and which usually assume an ideally infinitely high (insulation) resistance of the IT system against ground, is impossible in most cases, however, because of the relatively low-resistance HRG connection to ground in comparison to an IT system. Thus, a (low) loss of insulation in which a high-resistance insulation fault remains with the consequently low fault current cannot be detected. The state of insulation of an HRG system is monitored mainly by means of differential current measurement at the neutral-point connection. It proves disadvantageous in this regard that symmetrical faults, mutually compensating leakage and fault currents and crosscurrents flowing through ground between the active conductors cannot be detected, either, by means of the central differential current measurement of the neutral-point current.
Further, simulation results for a fault constellation in an HRG system having a capacitive crosscurrent via ground show that, in case of a fault current with a peak value of about 30 A due to the insulation fault (resistance), the measurable differential current via the neutral-point resistance is below a peak value of 5 A. This means that, even though the current flow in the neutral-point connection is limited, exactly those dangerous fires are caused that were supposed to be prevented by the current limitation in the neutral point.
Similarly as in IT systems, in case of a first insulation fault, the fault must be located and eliminated as quickly as possible in HRG systems as well. Methods are known from the state of the art for fault location in an HRG system in which a part of the neutral-point resistance is pulsed and periodically bridged. The periodically lowered neutral-point resistance value leads to a pulsed increase of the fault current value in the faulty outgoing power feed of the HRG system. This pulsed fault current can be detected by means of differential current measurement. However, this method for insulation fault location has the disadvantage that, as a passive method, it cannot be extended to monitor symmetrical losses of insulation.